IMO, running power meters aren’t very useful because they’re an indirect measurement and pace is a better standard anyway.
If HR is higher when running with weight, then the additional hrTSS is probably close enough.
Also, it makes sense that your pace would be slower and/or HR would be higher. 6kg is a lot to run with. As a comparison, a high-end skimo kit (including boots, skis, clothing, helmet, and pack) would be less than six kilos. It doesn’t take much extra weight to have an effect.